What if it's an imperfect recreation of the Mirror of Twilight? It can't bridge the two worlds, but it can allow echoes from one, weak simulacra, into the other?
We are led to believe the Dark Mirror was once the Mirror of Twilight. But what if it's the broken Mirror of Twilight recreated? Not capable of what it once was as a bridge, but instead a way to let some of that darkness energy in.
My latest Skyward Sword theory uses the idea of Hylia's time gates having access to a "memory update" timeline that she can use to try new tactics against Demise. The Triforce on the Gates tells me that perhaps Hylia-as-Zelda wished in order to create them. It took a Triforce wish to create these gates. And what the wish REALLY did, what ALL Triforce wishes do are system updates to "World of Hyrule" in the sense of if it's a computer simulation in memory. Certainly, A Link To The Past reflects that concept. This is from the metaphorical (not literal) idea from the Famicom original concept of Link as an entity living in a computer chip (like Tron). The Zelda franchise has always paid homage to this. The way the metaphor works is that the "gods" of the very real and normal Hyrule, are similar to the same computer programmers programming the world. So it's not literal, but the (cue Japanese culture) feeling or impression of the programmer making Hyrule in the game is the same as the gods making Hyrule.
So what Hylia's gate does is grant access to a "memory update" timeline that will eventually replace the old timeline, in set of endless time loops until Demise is defeated. This second timeline is the product of a Triforce wish. A "try it again" timeline.
The Mirror of Twilight also bears the Triforce symbol. This leads me to think that it was created as a Triforce wish. That the Twilight Realm is a world that is part of whatever overall universe simulation Hyrule is within, but one which is NOT supposed to be accessible. It's said that the two worlds near at dusk and dawn, and whatever relationship they do have, it's expressed loosely in these moments. But otherwise the two realms are completely disconnected.
So a Triforce wish could rewrite the rules, and grant access to a world that shouldn't be accessible in the "simulation". Hence, the mirror of twilight. You would have to imagine the wish takes the form of, "I wish there was a world I could send powerful beings that was so far from ours, there was no way for them to return." And the Mirror of Twilight becomes the object which receives power from this wish.
Once destroyed, you cannot recreate the Mirror without the Triforce. But perhaps some of its power remains to a degree.
